Where should specific parts be placed in PPR?

Hi,
I have a PPR context with 3d geometry of product (EBOM) and manufacturing product (MBOM). MBOM is defined from EBOM parts. But I want to add some additional parts to MBOM which are not related to product, e.g. mold, fixtures for some product parts. Where should I place these specific parts in PPR tree? And how can I assign it to MBOM?

Thanks